I would definitely recommend that seasonic x650. A top of the line product for a great price. Its like a level or more above the 80+ gold types, especially in efficiency. A comparable quality level product would be Enermax Modu87+ 700 at newegg for $200 with $25 mail in rebate.
I have looked and looked, there just isn't any other place to get this quality level for such a price. Ultra efficient, fanless operation till you get a pretty sizable load in it, then very quiet at first. Works just as well in higher case temperatures as room temperatures, modular of course.
Tests show it to get like 143w or so above its rating(probably not as ultra efficient then though). And in the future, you could just get more efficient energy cards and such. I prefer getting more powerful card, rather then duel card, its just so inefficient to SLI.(though I am sure that duel cards will get more efficient in the future)
Check out one of many glowing reviews on it. http://www.hardwaresecrets.com/article/Seasonic-X-Series-650-W-Power-Supply-Review/837/7